Changes in the building structure of wind turbines need to be monitored thoroughly as they indicate possible risks for the operation and safety of a wind turbine. Such changes comprise for instance damages to the tower of the wind turbine such as dents or developments of cracks or the like and the loss of pretensions in bolt assemblies within the wind turbine structure, in particular in the tower of the wind turbine. Furthermore it also concerns changes in the foundation which may be caused by earthquakes, erosion of the surrounding sand in an offshore foundation or cracks in the foundation. Other possible changes may occur in monopile foundations of wind turbines offshore. Other examples of changes in the wind turbine structure relate to yaw bearing problems and ice on the wind turbine. Most of these changes constitute a considerable risk for the wind turbine.
Monitoring such changes reliably is therefore an important task in order to avoid further damages and to make sure that the wind turbine has a long lifetime. So far detecting and tracking the changes in wind turbine structures is mainly done with non-automatic means. The wind turbines have to be inspected by staff on a regular basis and tests, for instance manual tests, have to be carried out at the site of the wind turbines in order to make sure that changes in their structure are found early enough to prevent further damage.